An important quality parameter for tablet production is maintaining the pregiven tablet weight, which, with sufficient mixing material assures maintaining the limits for the content of the active ingredient. The basis for any automatic control of a tabletting machine is the relationship between measured maximum pressing force of a tablet and the mass of the material filled into the mold (tablet weight). There exists, therefore, a direct relationship between the tablet weight and the pressing force necessary for production of controlled tablets. This relationship is utilized for the application of automatic monitoring and control equipment on rotary tabletting presses. A specific pressing force is assigned to each tablet weight for a tablet shape given in advance by the pressing tool and an established tablet height, as a function of the material to be pressed. If the quantity filled and therefore the tablet weight fluctuates with a constant tablet height, there is a change in pressing force resulting directly from this.
It is already known to monitor the established tablet weight by means of a computer-controlled fully electronically operating PID control (proportional-integral-differential control) via the mean value of pressing forces, which is determined from the individual sequential pressings (Korsch, Wolfgang: "Drug control--The new computer-controlled press force monitoring device for fully automatic tablet weight regulation on rotary presses" in the journal "Swiss Pharma" 2 (1980) No. 4, pp. 28-33). During automatic tablet production, the PID control continuously compares the measured pressing force with the desired pressing force. If there is a deviation between these two values, then the computer automatically determines the difference and activates the metering device, thus the desired pressing force, and the prescribed tablet weight is achieved. In addition, the parameters of weight, height, and hardness of the tablets are measured discontinuously, whereby deviations from the set values lead to establishing a new pressing-force set value. Upon fluctuations in height or hardness, the new set value for pressing force is controlled by a path height adjustment.
A process for quality control in the production of tablets is described in EP-0,261,358. According to this, samples are taken during the production of tablets, which consist of a great number of tablets, whose common weight actual value is compared with the weight set value, and in which the tabletting machine is subsequently controlled corresponding to the deviation between the actual value and the set value in order to bring the tablet weight to the set value. Prior to a subsequent control of the tabletting machine, the weights of individual tablets of the sample are measured and compared. For a determination of one or more tablets of the sample essentially deviating in weight from the set value, the statistics are corrected by a correction calculation and the corrected value is utilized for the subsequent control.
It is a disadvantage in the known method, that the parameters of weight, height, and hardness of the tablets are sequentially controlled, due to the high mutual dependencies and the very complex system. A change in the dosage not only causes a change in weight, but has effects on the hardness and the density of the tablets. Due to the individual controls that occur sequentially at large time intervals, conditioned by the system, there arises a large inertia in the control of deviations. The set value and the boundary value for the parameters must be rigidly given beforehand by the system user, which can lead to error-associated eliminating or accepting of tablets, as the case may be, when these values do not optimally correspond to the conditions.